Different schemes, methods and protocols, for communication between nodes in different types of networks are constantly developed to improve the characteristics of the particular network. Oftentimes older, less capable, protocols will have to live side by side with more advanced protocols as older equipment, employing older protocols are slowly phased out and replaced with newer more capable units. Other times different design characteristics may force the use of different protocols side by side for purpose of efficient network design.
In development of new methods, protocols and equipment this knowledge impacts the design as considerations, regarding the capability of the new protocols, methods or equipments to function in an effective manner side by side with the older ones, need to be taken.
For receiver equipment there is generally at least part of the sequence of operation where the receiver must search for the beginning of a valid transmission or message. This is the case most all of the time for multiple-access systems without central coordination, of which one example is IEEE 802.11 wireless LAN (WLAN). In such systems a station is unable to simultaneously transmit and receive messages, and is thus unable to determine if another station began transmission simultaneously and thereby corrupted both transmissions.
A remedy is to employ a so-called “listen before talk” policy based around time-slots. Before transmitting each station must listen for at least one time-slot. If no transmission begins during this time the station may begin transmitting a message at the beginning of the next time-slot. Since a station cannot switch instantaneously from reception to transmission, some time must be reserved for this at the end of the time-slot. For this reason a first time period, called “clear channel assessment time” (CCA time), is defined where other transmissions need to be detected, which leaves a remaining time period in the time-slot for the station to switch to transmission so that transmission can be started at the beginning of the next time-slot. Specifically, a station listening from the start of a time-slot must be able to indicate that it has detected a valid signal within the end of the CCA time with a defined probability set by the specific protocol or standard.
The length of the slot-time defines the granularity of the system, and a shorter slot time means that there is less “dead time” on the air while all stations wait for the right to transmit. However, these slot-times are also chosen based on the possibility to design receiver equipment of reasonably complexity which can meet the detection probability requirements within the given CCA time. For the detection of a signal in the presence of noise, there is a trade-off between the probability of detecting a signal that is present and the probability of a false alarm. A longer detection time allows the noise to be “averaged out”, reducing the probability of a false alarm. The detector can then be made more sensitive to the wanted signal.
For the 802.11b standard, operating in the 2.4 GHz frequency band, a slot time of 20 μs was defined, of which 15 μs is allowed for CCA. A transmission should be detected with a 99% probability within the CCA time.
For the 802.11a standard, using OFDM transmission in the 5 GHz frequency band, a slot time of 9 μs was defined, out of which 4 μs is provided for CCA. A somewhat more relaxed detection probability of 90% is allowed to compensate for the shorter CCA time.
The draft 802.11g standard improves the performance over the 802.11b standard by using OFDM modulation in the 2.4 GHz band. However, to be compatible with older 802.11b devices operating in the same band, the basic implementation uses the same 20 μs slot time, diminishing performance compared to 802.11a.
For networks where it is known that legacy devices are not present, it is possible to implement an optional “short slot time” of 9 μs. However, the receiver must be capable of detecting both OFDM transmissions as well as 802.11b Barker-preamble based transmissions within the short 4 μs CCA time. The 4 μs CCA time is a difficult challenge for detection of an 802.11b preamble.
In order to reach even the relaxed detection probability of 90%, the probability of false alarm rises to very high levels. High levels of false alarms damages system performance firstly by indicating that the medium is busy during the slot during which the false alarm occurs, preventing transmission. Secondly and the main problem is, however that the false detection also causes the station to switch into 802.11b reception mode. This prevents transmission of messages and blinds the station to other incoming transmissions, including OFDM transmissions, until a time-out occurs possibly 50-100 μs, or more, later. In a short slot-time system, the dominant transmission format is likely to be OFDM, so it is clearly undesirable to have an 802.11b detection mechanism preventing operation of the station for large periods of time due to false alarms.